Radio frequency (RF) transmitters typically suffer from non-idealities associated with a local oscillator (LO) and in-phase (I) and quadrature (Q) signal paths that degrade transmission performance. LO leakage includes unintended generation of frequency content in the transmitted signal at the LO frequency. Such leakage occurs through various mechanisms, including due to insufficient isolation in the up-converter mixer resulting in the LO frequency leaking from the LO port of the mixer to the output of the mixer, and from residual DC offsets in the IQ signal paths at the input of the mixer mixing with the LO frequency. IQ impairment involves unintended generation of image frequency content in the transmitted signal. Such impairment occurs as a result of phase and gain mismatches of the in-phase and quadrature signal paths and up-converter mixers.
FIG. 1(a) depicts an embodiment of a frequency domain view of a signal to be transmitted by the transmitter. The signal includes frequency content at a signal frequency. FIG. 1(b) depicts an embodiment of a frequency domain view of a signal transmitted by a transmitter suffering from LO leakage and IQ impairment. The transmitted signal includes intended frequency content at the upconverted signal frequency as well as unintended frequency content at the LO frequency and the image frequency.
Therefore, a need exists for circuits and systems to calibrate transmitters to correct for LO leakage and IQ impairments, preferably while minimizing any further non-idealities and/or required circuitry in a calibration path of a receiver of a transceiver containing the transmitter.